Previous methods for modulating the polarization rotation characteristics (and thus the net optical transmission) of a liquid crystal micro display in a projection display system uses electronics integrated into the display to directly control the voltages on the pixel elements. In these micro displays, the nematic liquid crystal, the most commonly used type of LC, responds to the RMS (root mean squared) values of the pixel voltages. In order to achieve gray-scale control of these displays it is necessary to modulate the individual pixel voltages. Generally there are two approaches to implementing this modulation: Analog or Digital.
Analog modulation methods were commonly used with earlier micro displays. However they are poorly suited to very high-density displays due to the small pixel size and difficulty of storing accurate analog voltages. This difficulty often translates into poor device yields and pixel non-uniformity. Because of this, the micro display industry increasingly uses digital modulation methods.
Digital modulation usually takes the form of either pulse width modulation PWM or duty factor modulation DFM. PWM schemes involve applying a voltage pulse to the LCD that is of fixed amplitude and variable width, where typically the width ranges from 0 to the entire frame period, corresponding to gray level from 0 to full-scale. PWM schemes can produce excellent gray-scale results and are inherently monotonic and independent of LC turn on and turn off times. However, they are very complex to implement in actual display systems, they require significant amounts of system memory having very high data rates and they may require a large number of data latches in the pixel if used for color sequential operation. Alternate methods of achieving PWM can reduce the pixel circuit complexity but at the expense of requiring extremely high data rates. In practice, PWM schemes are generally too difficult or expensive for use in micro displays and are not widely encountered.
DFM schemes are the most widely used form of digital LC modulation. In DFM, fixed-amplitude voltage pulses for each gray level bit are applied to the LC. Depending on the particular gray level to be displayed, there are typically several voltage pulses for driving a pixel during the frame time. There can be up to one-half as many pulses as there are gray level bits, with the widths of the individual pulses corresponding to the binary weights of the individual bits. As the name implies, in DFM the total additive durations of the pulses divided by the total frame time determines the duty factor of the voltage. The problem with this scheme is that it does not take into account the finite rise and fall times of the LC and particularly of the fact that the rise and fall times are often different from each other. This causes the actual RMS voltage to differ from the theoretical duty-factor calculated from the voltage alone. More seriously, this error depends on how many sets of rising and falling edges there are, and thus on how many pulses there are, which changes drastically as a function of the desired gray level. The result is that DFM schemes are generally nonmonotonic at a number of gray levels, which is a serious problem. A number of schemes have been developed to attempt to correct this non-monotonic behavior. None of these schemes are fully satisfactory and most require substantial increases in cost, in complexity, and in data rate.
A co-owned application, incorporated by reference and entitled “An optically addressed gray scale electric charge accumulating spatial light modulator,” U.S. Provisional Application No. 60/803,747, addresses several of the DFM issues. However, very fast LC switching speeds and pulsed illumination are required. In many display systems, very fast LC switching speeds and pulse illumination are not possible. There is a need for a LC driving method that is less complicated than PWM but overcomes the non monotonic behavior of most DFM driving method and doesn't require extremely fast LC response times.